In order to study many different types of genes and genetic elements, genetic procedures that can be used conveniently and rapidly are being developed. Intermeshed with this approach the specific subject of DNA transposition is being studied, both for a fundamental understanding of the process and as a basis for further genetic applications. Because of the ease of manipulations in E. coli, these studies are started in E. coli and are further engineered for applications in other organisms including yeast and higher eukaryotic cells. Many of the procedures used involve the construction of special vectors and transposons that can be used to fuse parts of well-characterized genes to the complementary parts of other genes. This gene fusion process can be used to identify gene segments or domains, and the resulting gene fusions can be used for further genetic and biochemical studies. To increase the versatility of gene fusions, new types of genes are being sought with properties different from the most commonly used gene, the E. coli B-galactosidase gene. These include genes for other hydrolytic enzymes which like B-galactosidase have a wide range of colorimetric substrates, as well as composite or hybrid genes between the B-galactosidase gene and other types of genes such as for aminoglycoside resistance or thymidine kinase. Studies and applications of transposable elements are focusing on the Tn3 and the bacteriophage Mu transposons. Of particular interest are the transposon terminal sequences where the transposase proteins bind, and the aberrant transposition structures that are formed when these terminal sequences are altered. New technical applications can be developed with transposon termini by incorporating between them various genetic entities such as promoters, structural genes, replicons, recombination sites, packaging sites, and transfer origins. The resulting transposable constructs have many possible applications, including the formation of various types of gene fusions, DNA cloning entirely in vivo, directed DNA cloning (walking), localized recombination, site directed DNA transfer, new types of genetic selections, and rapid DNA sequencing. Studies of aberrant transposition products will increase our understanding of the process of DNA transposition and may allow the development of new applications for transposable elements. Additional experiments proposed involve testing transposition of Tn3 and Mu elements in other species to extend their uses to higher species, as has been done with the B-galactosidase gene-fusion technology.